What is inheritance. 8: Inheritance. Recall the objectives of OOP. Defining a Subclass. A cylinder is like a circle, but with an extra length

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1 School of Computer Science, University of Birmingham Java Lecture notes. M. D. Ryan. October : Inheritance What is inheritance A way of defining a new in terms of an old one. Idea: B is like the existing A, except with the following differences:... We can add new variables and methods and also override the behaviour of existing methods. We say that B extends A. Class B inherits the properties of A. Recall the objectives of OOP To try to deal with the complexity of programming To apply principles of abstraction in order to simplify the tasks of writing, testing, maintaining and understanding complex programs To increase code reuse to reuse es developed for one application in other applications instead of writing new programs from scratch. Inheritance is a major technique for realising these objectives A.java public A... B.java Defining a Sub public B extends A... extra new features... A is the original super B is the new sub B inherits (or extends) A B is derived from A B is everything A is and more a B object is also an A object public Cat public void kick() System.out.println( Meeeooowww! ); public void stroke() System.out.println( Purr. Puurrr. ); public Lion extends Cat //Override the kick behaviour public void kick() System.out.println( RoooAAARRWW! ); //Lion also inherits the stroke method A cylinder is like a circle, but with an extra length The circle method findcircumference is still valid for cylinders. The circle method findarea will have to be changed if we want to find the whole surface area, but still, the old method is useful. r l r 1

2 public Circle private double radius; public Circle(double r) radius = r; public Circle() this(1.0); public double getradius() return radius; public double findcircumference() return 2*Math.PI*radius; public double findarea() return radius*radius*math.pi; public Cylinder extends Circle private double length; public Cylinder(double r, double l) super(r); length = l; public Cylinder() this(1.0,1.0); public double getlength() return length; continued... continued from prev. slide public double findarea() // overrides findarea in Circle return 2*super.findArea() + findcircumference()*length; public double findvolume() return super.findarea()*length; findarea overrides the findarea in Circle: it returns the total surface area of the cylinder. findvolume uses super.findarea to find the area of the cross-section. Points to note The sub adds variables and methods to what it inherits from the super. It may also call methods and constructors from the super. It may redefine methods rather than inheriting them from the super. method overriding. It can also redefine variables variable shadowing. Derived es: a hierarchy Circle Cylinder Object Student Person Lecturer the Object is very general, having just the methods equals and tostring (they are normally overridden) Employee Secretary this and super this refers to the executing object. this.varname is used to disambiguate a reference to a variable in the, when there is a method variable also called varname. this( ) is used in a constructor to call a constructor with a different signature. super is used similarly, to refer to the super. The first thing a constructor in a derived does is call a constructor in the base : super(). It does this whether you write it or not. 2

3 Example: car behaviour We want to model cars as follows: Black cars are rational (normal). Blue cars are also rational, but a bit slow. Red cars like to drive fast, but they break sharply when too close to the car in front. Green cars slow down at random, to admire the countryside. public Car public static final double SPEEDLIMIT = 7; private double speed; protected Color color; Car() speed = SPEEDLIMIT; color = Color.black; public double getspeed() return speed; public void setspeed(double s) if (s>=0 && s<=speedlimit+2) speed=s; //Speed limit not strictly enforced public void update(int d) double s = speed; s+= 0.05*(SPEEDLIMIT-s); // try to go a little faster if (s >= d) s=d-1; // but avoid crashing into car in front setspeed(s); public void updateroad() int distahead=99999; //Treated as infinity, no car in front //Update positions of cars based upon distance to next car ahead for (int i=road.length-1; i>=0; i--) if (road[i]!=null) //Update the car s speed road[i].update(distahead); //Move to new position on road int newpos = i+(int)road[i].getspeed(); if (newpos<road.length) road[newpos] = road[i]; if (newpos!=i) road[i]=null; distahead=0; else distahead++; //Add a new car to road if ( road[0]==null && r<arrivalprob ) road[0] = getnewcar(); Overriding Car s Behaviour Car BlueCar RedCar GreenCar // blue cars have lower max speed public BlueCar extends Car BlueCar() // super(); // this is implicit color = Color.blue; public void update(int d) super.update(d); // blue cars have lower max speed if (getspeed() > SPEEDLIMIT-3) setspeed(speedlimit-3); // red cars tend to drive fast, but // break sharply when close to car in front public RedCar extends Car RedCar() color = Color.red; public void update(int d) double s = getspeed(); // try to go a lot faster s+= 0.1 * (SPEEDLIMIT+3-s); // if near car in front, break sharply if (d < 8) s -= 3; // avoid crashing into car in front if (s >= d) s=d-1; setspeed(s); 3

4 // Green cars slow down to admire scenery. // With probability.01 they into a phase of slowing down over 12 cycles public GreenCar extends Car private int slowdown=0; // counts cycles during slowdown phase private final double SLOWSPEED = 0.2*SPEEDLIMIT; GreenCar() color = Color.green; public void update(int d) super.update(d); // if not already in slowdown phase, consider entering it if (slowdown==0 && getspeed()>6 && Math.random()<0.05) slowdown = 12; // if in slowdown phase, slow down towards SLOWSPEED if (slowdown>0) if (getspeed()>slowspeed) setspeed(slowspeed); slowdown--; private Car getnewcar() // pick a random number between 0 and 20 double rand = 20*Math.random(); if (rand<1.0) return new RedCar(); else if (rand<2.0) return new BlueCar(); else if (rand<3.0) return new GreenCar(); else return new Car(); Casting objects It is always possible to convert a sub to a super. In this case, explicit casting can be omitted. For example, Circle mycircle = mycylinder is equivalent to Circle mycircle = (Circle)myCylinder; Casting from super to sub Explicit casting must be used when casting an object from a super to a sub. This type of casting may not always succeed. Cylinder mycylinder = (Cylinder)myCircle; What is the "type" of a derived? Derived es have more than one type Of course they have the type of the derived (the they define) They also have the type of every ancestor all the way to the top of the hierarchy All es derive from the original, predefined Object In the cars example, all cars are of type Car (even those of type RedCar, etc). Polymorphism Polymorphism (or generic programming) is a technique that allows a single piece of code to be reusable with many different types. Cat clarence = new Cat(); if (Math.random>0.5) clarence = new Lion(); //The clarence variable is of type Cat c.kick(); //But this may kick a Cat or Lion object Which method is called is decided at runtime, not compile-time. 4

5 Polymorphism - Example Object[] myobjects = new Object[4]; //Fill the array with objects myobjects[0] = new Fraction(3,4); myobjects[1] = new Employee(); myobjects[2] = new Cylinder(); myobjects[3] = Java ; Polymorphism - Example public static void printarray(object[] objects) for (int i=0; i<objects.length; i++) Object o = objects[i]; System.out.print( Item at + i + : ); //Calls each object s tostring method //to print the contents of the array printarray(myobjects); printarray also accepts arrays of type String[], Fraction[] or arrays of any derived from Object. if (o==null) System.out.println( null ); else System.out.println( o.tostring() ); private / public / protected The final modifier private instance variables from the parent are not available by name in derived es. "Information hiding" says they should not be. use accessor methods. private methods are not inherited! use public to allow methods to be inherited. only helper methods should be declared private. protected means available to subes. A final cannot be extended: final Math... A final variable is a constant: final static double PI = ; A final method cannot be overridden by its subes. public final void update(int d); The abstract modifier The abstract method Method signature without implementation, intended to be implemented in sub. Useful to ensure that objects of sub can call the method. The abstract A must be declared abstract if any of its methods are abstract. Cannot be instantiated Should be extended and implemented in subes The instanceof Operator Use the instanceof operator to test whether an object is an instance of a : Circle mycircle = new Circle(); if (mycircle instanceof Cylinder) Cylinder mycylinder = (Cylinder)myCircle;... 5

6 Numeric wrapper es Boolean Character Short Byte Integer Long Float Double The Integer and the Double Constructors Class constants MAX_VALUE, MIN_VALUE Conversion methods Multiple inheritance? Interfaces interface "extends" "implements" Geometric Geometric interface Circle Cylinder Circle Cylinder circle cylinder circle cylinder Using interfaces Objective: Use the generic sorting method defined in the interface to sort an array of circles in increasing order of their radii and an array of cylinders in increasing order of their volumes. An interface public interface public static final int LESS = -1; public static final int EQUAL = 0; public static final int GREATER = 1; public int compare( otherobject); 6